This invention relates to rock drill bits and the materials used to fabricate them.
Earth boring drill bit bodies utilizing polycrystalline diamond compact (PDC) inserts are well known in the art. These PDC bit bodies are fabricated from either steel or a hard metal "matrix" material. The matrix material is typically a composite of macro-crystalline or cast tungsten carbide infiltrated with a copper binder alloy. However, these drill bit bodies encounter significant problems when drilling in certain earth formations. The steel bodies, for example, do not possess enough erosion resistance critical to many drilling applications. The matrix body, on the other hand, has a high erosion resistance, but its impact resistance is low, and its potential use may be limited.
Earth boring drill bit bodies are also manufactured by sintering, a process unique from infiltration. The sintering process involves the introduction of a refractory compound into a mold. The refractory compound is usually a carbide of tungsten, titanium or tantalum, with some occasional specialized use made of the carbides of columbium, molybdenum, vanadium, chromium, zirconium and hafnium. Before the carbide is introduced into the mold, it is mixed with a binder metal. The binder metal is usually cobalt, but iron and nickel are used infrequently. The percentage of cobalt typically ranges from three to fifteen percent. After the mixture of the refractory compound and binding metal is introduced into the mold, the combination is heated to a point just below the melting point of the binder metal, and bonds are formed between the binder metal and the carbide by diffusion bonding or by liquid phase material transport. Thus, sintering is the process of bonding adjacent metal powders by heating a preformed mixture.
Infiltration, on the other hand, involves the introduction of a refractory compound such as tungsten carbide, usually the carbides listed above, into a mold with an opening at its top. A slug or cubes of binder metal are then placed against the refractory compound at the opening. The mold, refractory compound and binder metal are placed into a furnace, and the binder metal is heated to its melting point. By capillary action and gravity, the molten metal from the slug infiltrates the refractory compound in the mold, thereby binding the refractory compound into a part. As stated above, the infiltration binder is typically a copper alloy. Specifically, the composition of the binder is copper alloyed with nickel, manganese, zinc, tin, or some combination thereof.
The copper infiltrated tungsten carbide drag bit body possesses high wear resistance and, because of the hardness of the carbide, high erosion resistance as compared to steel, but the strength of the composite is poor in terms of either the charpy impact strength test or the transverse rupture strength test. Examination of failed bit bodies reveals the failure occurs between the copper to carbide bond. Thus, the tungsten carbide bonded with the copper alloy has low strength properties because failure occurs at the connection between the copper and the carbide, not within the copper alloy. A conventional copper matrix bit in a charpy test breaks at approximately 30 inch pounds and has a transverse rupture strength of 100 ksi. Thus, the copper infiltrated tungsten carbide drag bit body has overcome the wear and erosion resistance problems of the steel earth-boring drill bit bodies, but it would be desirable to overcome the reduction in strength that occurs in the tungsten carbide bonded with a copper alloy. Though the increased wear and erosion resistance provides an increase in the life of the drag bit body, increasing the strength limitations of the copper infiltrated tungsten carbide drag bit bodies without reducing the wear and erosion resistance would lead to a reduction in the number of round trips of a drill string in a borehole and increase in the rate of penetration of bits into the rock formation. With a stronger bit body, higher weight may be applied to the bit to provide faster penetration.
Thus, increase in the strength of the PDC bit body, while maintaining wear and erosion resistance, is desirable to reduce round trips, enhance the rate of penetration for the drag bit, and increase the possible variety of body designs and insert configurations. Such increases in the versatility of designs and in the rate of penetration, and decrease in round trips, translate directly into a reduction in drilling expenses.